Memory devices using semiconductor elements are broadly classified into two categories: a volatile device that loses stored data when power supply stops, and a non-volatile device that retains stored data even when power is not supplied.
A typical example of a volatile memory device is a dynamic random access memory (DRAM). A DRAM stores data in such a manner that a transistor included in a storage element is selected and electric charge is accumulated in a capacitor.
When data is read from a DRAM, charge in a capacitor is lost according to the above-described principle; thus, another writing operation is necessary whenever data is read out. Moreover, since leakage current (off-state current) or the like flows between a source and a drain of a transistor included in a memory element when the transistor is in an off state, charge flows into or out even if the transistor is not selected, which makes a data holding period short. For that reason, another writing operation (refresh operation) is necessary at predetermined intervals, and it is difficult to sufficiently reduce power consumption. Furthermore, since stored data is lost when power supply stops, an additional memory device using a magnetic material or an optical material is needed in order to hold the data for a long time.
Another example of a volatile memory device is a static random access memory (SRAM). An SRAM retains stored data by using a circuit such as a flip-flop and thus does not need refresh operation. This means that an SRAM has an advantage over a DRAM. However, cost per storage capacity is increased because a circuit such as a flip-flop is used. Moreover, as in a DRAM, stored data in an SRAM is lost when power supply stops.
A typical example of a non-volatile memory device is a flash memory. A flash memory includes a floating gate between a gate electrode and a channel formation region in a transistor and stores data by holding electric charge in the floating gate. Therefore, a flash memory has advantages in that the data holding period is extremely long (almost permanent) and refresh operation which is necessary in a volatile memory device is not needed (e.g., see Patent Document 1).
However, a gate insulating layer included in a storage element deteriorates by tunneling current generated in writing, so that the storage element stops its function after a predetermined number of writing operations. In order to reduce adverse effects of this problem, a method in which the number of writing operations for storage elements is equalized is employed, for example. However, a complicated peripheral circuit is needed to realize this method. Moreover, employing such a method does not solve the fundamental problem of lifetime. In other words, a flash memory is not suitable for applications in which data is frequently rewritten.
In addition, high voltage is necessary in order to inject charge into the floating gate or remove the charge, and a circuit for generating high voltage is also necessary. Further, it takes a relatively long time to inject or remove charge, and it is not easy to perform writing and erasing at higher speed.
Another example of a non-volatile memory device is a magnetoresistive random access memory (MRAM) which is a memory device including a magnetic material. An MRAM consumes a comparatively large amount of current in writing operation; therefore, there is a problem in that it is difficult for an MRAM to perform writing operation in a plurality of memory cells at concurrently.